1. Field of the Invention
The present invention relates generally to liquid junction structures for electrochemical electrodes and, more particularly, to liquid junction structures particularly adapted for glass electrode bodies.
2. Description of the Prior Art
In general liquid junction structures are incorporated in electrochemical electrodes which measure ions in solution in order to establish electrolytic contact between the solution to be tested and an electrolyte within the electrode. The liquid junction structure should establish electrolytic contact between the two solutions without introducing an extraneous electrical potential generally referred to as a liquid junction potential for a reference electrode. With an ideal liquid junction the potential of a reference electrode remains constant and is substantially independent of the solution being tested. The ideal characteristics of a liquid junction are well defined in the art. Though numerous liquid junction structures have been developed, refined, discarded and rediscovered, and such over the years, the search continues for further structures approaching the elusive ideal characteristics.
Among the ideal characteristics, a liquid junction structure should establish electrolytic contact by diffusion or, if by flow, then at a very even, continuous, low flow rate. The liquid junction should have a low or negligible impedance, should resist clogging, should be easily wet despite its low flow rate, and should be easily cleanable. In addition, the junction should not generate potentials dependent upon the sample composition.
U.S. Pat. No. 3,264,205 discloses a liquid junction structure comprising a porous ceramic coating which is particularly adapted for glass electrode bodies. The ceramic coating is readily fired onto the glass body. In one embodiment the patent discloses an outer tubular glass body having such ceramic coated around the inner surface of one open end. The open end is closed by a rubber stopper and the liquid junction path is defined through the porous ceramic coating along the length of the rubber stopper to the electrolyte reservoir inside the glass body. An inner glass pH measuring electrode extends through the stopper coaxially within the outer glass body, and the patent teaches that the ceramic could in theory be coated around the outer surface of the glass pH electrode to establish an alternative liquid junction path through the ceramic along the pH electrode for the full length of the rubber stopper. Where a flowing liquid junction structure is employed, i.e. where a slight head pressure induces liquid flow through the junction, the relatively long length of the foregoing porous ceramic junction is not a serious advantage. However, where an ionic diffusion junction, as opposed to a flowing junction, is desired the long junction path is a definite disadvantage. Diffusion junctions are employed in non-refillable type electrodes where depletion of electrolyte is to be avoided. Such non-refillable electrodes are sealed at manufacture and are discarded when the electrolyte is depleted. In a diffusion junction there is no flow and the diffusion of ions through the solution takes place at an extremely slow rate. Consequently, use of such an electrode in different solutions will allow various sample solutions to leach out the ions of the reservoir electrolyte from the junction and replace them with sample solution components.
The ions of the reservoir electrolyte are specially chosen so that the cations have the same mobility as the anions. These ions are also present at relatively high concentrations in the reservoir electrolyte so any contamination by samples will have a minimal effect on the mobilities of cations and anions. This arrangement minimizes liquid junction potentials at the reference electrode. With a flowing junction the reservoir electrolyte is constantly flowing through the liquid junction. This prevents the ingress of sample components and keeps the junction filled with reservoir electrolyte. With a diffusion junction, on the other hand, when it is immersed in different solutions the ions of the reservoir electrolyte can diffuse into the sample solution and be replaced by components of the sample. Since these ions will probably not have equal mobility of cations and anions, a liquid junction potential can arise. This effect can be counteracted by diffusion of ions from the reservoir into the liquid junction since the concentration of reservoir electrolyte is relatively high. Since the diffusion process is very slow the replenishing of electrolyte in the liquid junction cannot take place if the junction is thick or long.